The NAND flash memory has been designed to increase the coupling ratio of memory cells by way of increasing the film thickness of the floating gate, which is the charge storage layer, and increasing the contact surface area between the floating gate and the inter-poly dielectric film (the IPD film). However, as downscaling progresses and the distance between adjacent memory cells decreases, the fluctuation of the threshold voltage becomes exceedingly large due to “proximity effects”, i.e. the capacitive coupling between the floating gates of adjacent memory cells; and downscaling has become difficult.